Because they’re content scrapers.
Plagiarism 1.0 — handwriting
Have you ever seen someone in school write an essay by copying out an encyclopaedia article by hand? It’s unethical and very tedious.
Plagiarism 2.0 — copy and paste
Computers are good at automating tedious tasks. Find a web page on the topic, copy it, paste it, print it, and hand it in. This will free up plenty of time for practising your best “innocent” expression in front of a mirror. You will need it when your teacher runs a web search using phrases from your essay1 and finds the exact site you stole it from.
Plagiarism 3.0 — more copy and paste
If your innocent expression is good enough, you can pass high school — or even University — without learning any skill other than copying and pasting. What can you do with that skill?